The final sensor we’ll look at is the AD22151 magnetic field sensor by Analog Devices. Its primary use is for position and proximity sensing. A magnetic source is used as a reference point, and the sensor’s distance from that source may be easily determined by the measured field strength. The sensor has built-in temperature compensation and amplification.
The circuit for this sensor is shown in Figure 12-20. It’s a little bit more complicated than the other sensors we’ve looked at so far.
The sensor operates off a 5V supply, decoupled to ground using a 100nF capacitor. There are four resistors required for correct operation. R1 is the temperature compensation resistor. R1 should be connected between pins 1 and 3 or pins 2 and 3, depending on the applied magnetic field. For large external fields, R1 connects pins 1 and 3, as shown in Figure 12-20. For smaller fields, connect R1 between pins 2 and 3. The AD22151 datasheet has plots of values for R1 versus required compensation levels. Check with the manufacturer of your magnetic source[19] as to the required compensation value, and use this in conjunction with the datasheet to determine R1.
For magnet data, try http://www.magtech.com.hk, http://www.eastindustries.net, or http://www.millennium.com.hk as places to start.
R2 and R3 set the signal gain of the internal amplifier, and R4 provides a voltage offset. The datasheet for the sensor contains equations and technical data for computing values of these resistors, based upon your specific needs.
The output of the sensor circuit may be connected directly to an ADC input for sampling.
[19] I’m assuming here that you are using a magnet specifically intended for such applications and which has data available, rather than a magnet you’ve found lying around somewhere.